1. Field of the Invention
This invention relates to an antenna module and an electronic device using the same and, more particularly, to an antenna module with a small area and an electronic device using the same.
2. Description of the Related Art
In recent years, with fast development of global handheld wireless communications industry, cell phones are developed from dual-band (such as a global system for mobile communications 900 (GSM900)/a digital cellular system (DCS)) cell phones with a simple telephoning function in an early period to tri-band (such as GSM900/DCS/personal communications service (PCS)) and even to quad-band (such as GSM900/DCS/PCS/wideband code division multiplex access (WCDMA)) cell phones with a video telephoning function. Since functions of the cell phone are gradually oriented to audio-visual entertainment, such as photographing, digital music listening, a wireless network, a global positioning system (GPS) positioning, a mobile television function and so on, volume of circuits gradually increases. However, to satisfy aesthetic, the size of the cell phone is gradually reduced, and relatively space of an antenna is usually reduced. Therefore, metal electronic components, such as a camera, a speaker, a vibrator and so on, are disposed under the antenna, and the metal electronic components may interfere with the antenna, such that signal reception quality of the antenna may also become worse due to a bad environment of the antenna.
FIG. 1 is a side view showing a conventional planar inverted-F antenna 100. In FIG. 1, the conventional planar inverted-F antenna 100 includes a metal ground surface element 101, a radiation conductor element 102, a short-circuit conductor element 103, and a signal feeding part 104. The appearance of the side structure of the planar inverted-F antenna 100 is the same as that of an inverted letter F, therefore, the antenna is named planar inverted-F antenna (abbrev., PIFA). A terminal of the radiation conductor element 102 of the antenna 100 is a short-circuit terminal formed by being connected with the metal ground surface element 101 via the short-circuit conductor element 103 at its tail terminal which is a quarter-wavelength microstrip line far from its open-circuit terminal, and the signal feeding part 104 is located between the radiation conductor element 102 and the metal ground surface element 101. Impedance matching can be achieved by adjusting a distance between the signal feeding part 104 and the short-circuit conductor element 103 to find resistance 50 Ω of a signal feed-in resonant point, and at that moment, the value of reactance ideally approximates to zero, thereby achieving the better impedance matching to excite electromagnetic radiation to transmit a signal. However, since a capacitance effect can be generated between the metal ground surface element 101 and the radiation conductor element 102, and a capacitor can store energy, the capacitance effect can reduce the energy transmission efficiency of the antenna such that a part of the energy is stored by the capacitor and the energy cannot be wholly radiated out.
Generally speaking, when a size of a conventional antenna of a cell phone is 40.0×20.0×7.0 mm3, the four bands GSM900/DCS/PCS/WCDMA can be wholly covered. Therefore, the conventional antenna is unfavorable for light, slim, short, and small development of the cell phone. Besides, since the size of the antenna is great, the metal electronic components may fail to keep away from the antenna thus to interfere with the antenna and to affect the signal reception quality.